In Hitman 2, Level Mastery is the measure of your completion for a given stage. Each location has its own Mastery Challenges and levels to earn, with the amount of level ranging from 5-20, but to increase Mastery in Hitman 2 you’re going to have to basically complete the objectives that the game lays out for you.
Within each location in Hitman 2 you will have Challenges to complete. There are Assassination Challenges, Discovery Challenges, Feats to complete, targets to take down, and classic challenges to complete. Each challenge that you complete will have a different amount of Mastery Points associated with it. Earning Mastery Points will go towards increasing your Mastery Level, and at each level there will be a new item unlocked for you.
These Mastery Unlocks include things like weapons, tools, and other items that can aid you in your assassination plots. There are other Mastery Level unlocks as well, these include new Starting Locations, Agency Pick-up locations, and Hidden Stashes where you can store weapons.
The best weapons and unlocks are usually reserved for the higher mastery levels.
To increase mastery you can plan ahead and see which challenges will offer you the most Mastery Points. This can be done from the main menu out of mission. From the main screen just tab over using the R1/RB button to the Career Tab and then select challenges. There you can see just how much each challenge will increase your Mastery Level.